This invention relates to optical filament transmission media and, more particularly, to high bandwidth optical waveguides.
Waveguides used in optical communication systems are herein referred to as optical waveguides, and are normally constructed from a transparent dielectric material, such as glass or plastic.
Gradient index optical waveguides have a radially varying composition and consequently a radially varying refractive index. Reference is made to U.S. Pat. Nos. 3,823,995 to Carpenter and 3,711,262 to Keck and Schultz as examples of gradient index optical waveguides as well as examples of formation of optical waveguides by inside vapor phase oxidation processes. Both of these patents are expressly incorporated herein by reference. The inside vapor phase oxidation processes include chemical vapor deposition, flame hydrolysis and any other processes by which vaporous material is directed into a heated tube, reacted with oxygen under the influence of heat and deposited on the inside wall surface of said tube. The material is deposited within the tube in successive layers and the tube is then removed from the heat to leave a fused blank. As will be understood, the central hole may be collapsed at the end of the deposition process, the blank may subsequently be reheated and the hole collapsed, or the hole may be collapsed during the drawing process. In any event, the blank or preform is subsequently heated and drawn into an elongated, fine strand. Inasmuch as the structure of the drawn strand or filament reflects the structure of the drawing blank or preform, it is important that the physical characteristics of the blank be carefully controlled.
As is familiar to those skilled in the art, gradient index optical waveguides commonly exhibit a higher index of refraction in the core at center thereof; and lower indices of refraction at points radially outward from the core center. The gradient index of refraction may, however, be varied continuously or discontinuously, and may exhibit a linear, parabolic, or any other desired characteristic. Information concerning the construction and use of optical waveguides may be found in "Fiber Optics Principles and Applications" by N. S. Kapany, Academic Press, 1967; "Geometrical Optics of Parabolic Index Gradient Cylindrical Lenses" by F. P. Kapron, Journal of the Optical Society of America, Vol. 60, No. 11, pages 1433-1436, November, 1970; and "Cylindrical Dielectric Waveguide Mode" by E. Snitzer, Journal of the Optical Society of America, Vol. 51, No. 5, pages 491-498, May, 1961.
In order to effect such change of the index of refraction of a blank or preform being formed by an inside vapor phase oxidation process, the chemical composition of the source materials, which, after reaction, comprise the ultimate material deposited on the inside surface of the tube, may be varied. The vapor mixture is hydrolyzed or oxidized and deposited on the inside surface of the tube and subsequently fused to form a high quality and purity glass. At the same time, one or more additional vapors can be supplied to the tube, each vapor being constituted of a chemical termed a "dopant" whose presence affects the index of refraction or other characteristics of the glass being formed.
In general, optical waveguide blanks or preforms formed by the inside vapor phase oxidation process include forming a barrier layer on the inside of the support or substrate tube prior to the deposition of the core glass with the substrate tube being the cladding. The principal function of the barrier layer is to minimize interface scattering and absorption losses by removing the core-cladding interface which would exist between deposited layers of high purity, low attenuation glasses and the substrate tube inner surface. The barrier layer is conventionally a borosilicate glass composition since doping silica, which is generally the base glass, with boron reduces the deposition temperature and thereby minimizes shrinkage of the substrate tube. Other advantages of doping silica with boron is that it reduces the refractive index of the glass and it acts as a barrier to the diffusion of hydroxyl ions, commonly referred to as hydrogen or water, from the substrate tube to the deposited core glass at the elevated processing and drawing temperatures.
It is well known that the information bandwith of an optical waveguide filament can be substantially increased by grading the index of refraction profile. The bandwidth of a graded index of refraction optical waveguide can be from about 10 to 10.sup.3 times greater than the bandwidth of a filament with an ungraded index profile. The increase in bandwidth is very dependent on the shape of the index of refraction profile.
In the prior art formation of inside vapor phase oxidation gradient index optical waveguide preforms or blanks and the subsequent glass filaments, the filaments exhibit a combination step-graded index of refraction profile which causes pulse spreading of higher order modes resulting in lower bandwidth. Such a profile is illustrated in FIG. 1 where the gradient portion is indicated by curve 10 while the stepped portion is indicated by the substantially straight vertical portion 12 showing a step increase in the index of refraction.
Although not known to exist in the prior art, another example of a combination step-graded index of refraction profile which is believed to cause pulse spreading of higher order modes is illustrated in FIG. 2 where the gradient portion is indicated by curve 14 while the stepped portion is indicated by step 16, also showing a step increase in the index of refraction.
There are at least two causes of the step-graded profile illustrated in FIGS. 1 and 2. A high boron level is desired in the barrier layer for reasons discussed above. A low boron level is desired in the core to increase the numerical aperture of the resulting optical waveguide, to minimize Rayleigh scattering and to eliminate the infrared absorption of the B-O vibrational bands in the spectral range between 1.2 and 1.5 micrometers. (H. Osanai et al., Electronic Letters 12, 549, 1976.) The second reason for such a step-graded profile is that at the barrier layer-core interface a dopant such as GeO.sub.2 and/or P.sub.2 O.sub.5, or the like, must be introduced at a finite level set by the capability of the source material vapor delivery system. It should be noted that in the illustrations of both FIGS. 1 and 2, the index of refraction of the core at the barrier layer-core interface is higher, by a value of .DELTA.n, than the index of refraction of the substrate tube material or cladding. It is such a step increase in the core index of refraction at the barrier layer-core interface that is believed to produce the pulse spreading of higher order modes resulting in lower bandwidth. In each example, silica is shown as the substrate tube material having an index of refraction of 1.4570, with the index of refraction at the central axes 18 and 20 of the filaments of FIGS. 1 and 2 respectively being 1.4766. As will be understood, the indices of refraction are at a wavelength of about 630 nm. for a filament having a numerical aperture of about 0.24.
Curve 48 of FIG. 12 illustrates the pulse broadening which has been observed in a typical step-graded profile made by the prior art. The tall narrow portion of the pulse is produced by the graded part of the index of refraction profile, while the wide base extending to the right is caused by the step part of the profile. The bandwidth of the filament illustrated by curve 48 of FIG. 12 has been measured to be 260 mHz for a one kilometer length.
The method of the present invention avoids formation of the step part of the profile and results in the fabrication of pure graded index of refraction profiles such as those illustrated in FIG. 6 or 7. As illustrated by curve 52 of FIG. 12, a filament formed by the method of the present invention would have much less broadening and an estimated bandwidth of about 910 mHz for a one kilometer length. The reduced pulse broadening and high bandwidth are achieved by elimination of the step portion of the step-graded profile. In addition, the numerical aperture is increased, the Rayleigh scattering reduced, and the infrared absorption of the B-O vibrational bands in the spectral range between 1.2 and 1.5 micrometers is reduced.
A commonly used method of fabricating, for example, GeO.sub.2 -SiO.sub.2 -B.sub.2 O.sub.3 or SiO.sub.2 -GeO.sub.2 B.sub.2 O.sub.3 -P.sub.2 O.sub.5 core gradient index optical waveguides is illustrated in FIGS. 3 and 4 wherein the simultaneous reduction of the B.sub.2 O.sub.3 level and the introduction of finite levels of GeO.sub.2 alone and/or P.sub.2 O.sub.5 causes a step increase in the refractive index at the edge of the core and leads to the step-gradient profile of FIG. 1 as hereinabove described. On the other hand, the method illustrated by FIG. 5, showing the second type of undesirable step-gradient profile illustrated in FIG. 2, results when finite levels of GeO.sub.2 and/or P.sub.2 O.sub.5 are used in the barrier layer together with B.sub.2 O.sub.3, and the amount of B.sub.2 O.sub.3 in the barrier layer is insufficient to compensate for the increase in the refractive index due to the amount of GeO.sub.2 and/or P.sub.2 O.sub.5 present. In such a situation, a step increase in the index of refraction is caused at the cladding-barrier layer interface which leads to the step-gradient index profile of FIG. 2.
As will be noted, FIGS. 3, 4 and 5 illustrate the starting source materials namely the chlorides or the like of boron, silicon, germania, and phosphorous. As will be understood these source materials, under the influence of oxygen and heat, react to produce the respective oxides. As used herein, inside vapor phase oxidation includes "chemical vapor deposition" and other vapor phase oxidation methods. The phrase "chemical vapor deposition" means the formation of deposits by chemical reactions which take place on, at, or near the deposition surface, a definition set forth on page 3 of the text "Vapor Deposition" edited by C. F. Powell et al., New York, John Wiley & Sons, Inc., 1966, which text is hereby wholly expressly incorporated by reference. Any of the procedural variations well known in the art may be employed to effect the deposition of the suitable coating of glass by the chemical vapor deposition process, such as, for example that described on page 263 of the aforementioned Powell et al. text which states: "Another means of obtaining uniform coverage which also can give high deposition efficiency, and which is especially applicable to the coating of the inside surfaces of small bore tubing, is to heat only a small portion of the tubing to the deposition temperature . . . . The section heated to the deposition temperature is slowly moved over the total length of the tube or the total area to be coated." In this connection, reference is also made to U.S. Pat. No. 3,031,338 issued on Apr. 24, 1962 to R. G. Bourdeau.
Another effective means of applying coatings by vapor phase oxidation is to sinter a soot layer of the desired material applied by flame hydrolysis process similar to that described in U.S. Pat. No. 2,272,342 issued to J. F. Hyde or U.S. Pat. No. 2,326,059 issued to M. E. Nordberg, both of which patents are expressly incorporated herein by reference.